Currently in commercial aircraft so-called air-assisted air conditioning systems are conventionally used to air-condition the aircraft cabin. The aircraft air conditioning system is used to cool the aircraft cabin, which would otherwise become overheated as a result of thermal loads, such as for example insolation, body heat of the passengers and waste heat from equipment on board the aircraft. The aircraft air conditioning system moreover supplies enough fresh air into the aircraft cabin to ensure that there is a prescribed minimum oxygen content in the aircraft cabin. Finally, the aircraft air conditioning system is used during cruising of the aircraft above a specific altitude to keep the pressure in the aircraft cabin at a level higher than the ambient pressure.
In an aircraft air conditioning system known for example from DE 10 2006 016 541 A1, hot process air that is tapped from the aircraft engines during cruising of the aircraft is supplied to two air conditioning units, which are operable independently of one another. In the air conditioning units the process air, which is supplied at a high temperature and a high pressure, is conditioned in such a way that it leaves the air conditioning units as expanded and cooled process air. The expanded and cooled process air is supplied as fresh air to a mixer, in which it is mixed with recirculation air removed from the aircraft cabin. The recirculation air is conventionally conveyed by recirculation fans from the aircraft cabin into the mixer. The air mixture of fresh air and recirculation air that is produced in the mixer is finally conveyed into the aircraft cabin to ventilate the aircraft cabin.
If in flight a defect in the aircraft air conditioning system arises, as a result of which the aircraft cabin can no longer be supplied with sufficient fresh air, the aircraft drops to a safe altitude, at which it is no longer necessary to keep the pressure in the aircraft cabin at a level higher than the ambient pressure, and flies in an unpressurized state to the destination airport and/or to a less remote airfield. In order to provide the necessary breathable air for the passengers during this period, it is known to provide aircraft with one or more emergency ram-air inlets. By suitably controlling emergency ram-air inlet flaps, which during normal operation of the aircraft close the emergency ram-air inlets, it is possible to ensure that ram air from the aircraft environment is supplied through the emergency ram-air inlets and fed as fresh air into the mixer of the aircraft air conditioning system.
A fault scenario, in which a leak is generated, for example by detached rotor parts or the like, in the area of the mixer of the aircraft air-conditioning system, is to be categorized as extremely problematical. In the event of a leak in the area of the mixer, air escapes from the mixer into an aircraft area adjacent to the mixer. This leads to a pressure drop in the mixer, with the result that it is no longer possible to discharge enough air from the mixer into the aircraft cabin. The pressure drop in the mixer leads to a temporary rapid increase of the air-mass flow from the air conditioning units because of the diminishing mixer back-pressure. The controller of the air conditioning units then reduces the air-mass flow out of the air conditioning units, thereby leading to a further drop of the mixer pressure. A further effect of the pressure drop in the mixer is that the recirculation fans suck air out of the mixer instead of out of the aircraft cabin. A short-circuit in the recirculation air system consequently arises, as a result of which the recirculation air flow also breaks down.
In the fault scenario of a leak in the area of the mixer, therefore, neither enough fresh air from the air conditioning units nor enough recirculation air is directed into the aircraft cabin.
In the case of a leak in the area of the mixer, the fresh air supply through the emergency ram-air inlets is similarly unable to guarantee an adequate supply of fresh air to the aircraft cabin because the fresh air supplied through the emergency ram-air inlets is distributed in the cabin by means of the mixer. Consequently, the fresh air supplied through the emergency ram-air inlets also escapes unused into the surroundings of the mixer in the underfloor area of the aircraft. A leak in the area of the mixer of the aircraft air conditioning system may therefore lead, particularly in aircraft, in which no cockpit- or cabin window may be opened, to the CO2 content of the cabin air rising to a level that is injurious to health.